1. Field of the Invention
This invention relates to shaping glass sheets and has special utility for shaping glass sheets into certain cylindrical and conical shapes that are either used as components of laminated aircraft transparencies or as molds for press polishing shaped transparent components of plastic composition that are incorporated in aircraft transparencies. A glass sheet shaped to a particular cylindrical configuration produced according to the present invention either forms a press polishing mold for press polishing a transparent element of a laminated transparency, for example one found in the B1 aircraft, or forms a component to be laminated to other layers of glass and/or plastic material to form a laminated transparency. Another embodiment of the present invention provides a glass sheet of conical shape that is either used to press polish plastic components of conical shape to be laminated to form a laminated transparency, of which parts for the F-111 aircraft are typical, or for a component to be incorporated in conically shaped laminated transparencies that contain a glass component layer.
The number of parts that have to be shaped for use in aircraft transparencies or for making press polishing molds used to press polish aircraft transparency components is not so great as to warrant the use of high production equipment used to shape windshields, backlights and curved sidelights for automobiles. Therefore, parts destined for use in fabricating aircraft transparencies, either directly (as in components to be laminated) or indirectly (as in shaped molds to press polish components), are usually shaped during fabrication by gravity sagging in a kiln.
In the usual process of gravity sagging, glass sheets are heated to a deformation temperature, which varies for different thicknesses of glass from about 1050.degree. F. (approximately 566.degree. C.) to about 1200.degree. F. (approximately 649.degree. C.). For example, commercial float glass about 1/4 inch (6.35 millimeters) thick has a deformation temperature of about 1080.degree. F. (approximately 582.degree. C.) and thicker sheets of the same composition having a thickness of three-quarters of an inch (19 millimeters) have a deformation temperature of more than 1200.degree. F. (which approximates 649.degree. C.).
Components for large aircraft transparencies and molds for press polishing said components are usually shaped in a batch type process. In the batch process, a batch of one or more glass-laden molds is introduced into a furnace or kiln which is heated gradually to a temperature sufficient to cause the glass to sag into conformance with the shaping surface of the mold.
In bending glass sheets to cylindrical or conical shapes, a flat glass sheet is supported on a pair of oblique slides located beyond either side of a mold onto which the glass sheet sags by gravity when heated to its deformation temperature. The mold has an upward facing surface conforming to the outline contour and elevational shape desired for the shaped glass sheet. It is convenient that the flat glass sheet slide and sag at both of its opposite ends at an equal rate so as to be in proper alignment with the shaping surface of the mold upon which it is received. To accomplish this end, it is necessary to maintain the sheet in proper alignment with the surface that supports the glass in sliding relation thereon during its conversion from a flat shape to its ultimate shape so that its rate of sliding is uniform from one side edge to the other side edge. Unless non-uniform sliding of the glass sheet between its opposite edges is prevented, the sagging sheet deviates from its desired shape when finally supported on the outline mold.
A uniform temperature must be maintained within the furnace or kiln in which shaping takes place. Unless a uniform temperature is maintained, the glass sheet will slide by a combination of heat sagging and gravity along one obliquely disposed sliding surface more rapidly than the other obliquely disposed sliding surface. Under circumstances where the temperature in the kiln or furnace is not uniform, one edge of the glass sheet is likely to slide more rapidly down the slide and arrive at the mold shaping surface more rapidly than the side edge of the glass sheet opposite the first side edge arriving at the mold surface. The shaped glass sheet thus may either fall off the mold if the latter is of the outline type or will slide to the point where it is non-uniformly shaped with respect to the center of the glass. Such non-uniform shaping makes it difficult if not impossible to laminate the shaped glass sheet to other plies in forming a laminated transparency for aircraft.
2. Description of the Prior Art
U.S. Pat. No. 2,377,849 to Binkert and Jendrisak discloses a gravity sag mold having an air passage through which vaccum is applied by suction to enable the glass sheet to sag into conformity with the shaping surface of a gravity sag-type bending mold without forming air pockets between the lower surface of the glass and the upper surface of the mold. The elimination of air pockets allows the glass to conform closely to the shape of the mold.
U.S. Pat. No. 3,560,182 to Golightly discloses a gravity sag bending mold that provides a series of horizontally extending circumferential ridges extending in a horizontal plane in an inward direction from a slide member disposed over a support ring. The purpose of the ledges is to retard the sliding of a portion of the periphery of the glass that causes tilting because of non-uniform sliding at different portions of the periphery. This non-uniform sliding may be caused by non-uniform heating of the glass or many other causes, such as the glass being of non-uniform thickness so that the thicker regions tend to resist sliding to a greater extent than the thinner regions, or different portions of the kiln or furnace may be hotter than other portions so as to impart a non-uniform heating pattern onto the glass.
In the case of a continuous bending operation where the glass supporting mold moves continuously through a lehr that has an increasing temperature gradient along the path of glass travel, the leading edge of the glass is continuously exposed to a higher temperature than the trailing edge throughout its movement through the lehr and softens more rapidly. A hotter portion of the glass sheet marginal portion is more fluid and, hence, slides more readily than a colder portion. This non-uniformity in sliding would cause the glass to tilt away from its desired orientation relative to the mold shaping surface. Tilting of the glass promotes deviation from the desired shape.
Whatever the reason may be for the glass sagging non-uniformly, the Golightly invention remedies this problem by mechanical means that interrupt the free sliding of a portion of the periphery that reaches one of several pre-determined intermediate locations between the original position occupied by the flat glass sheet when first mounted over the mold for shaping and the final position the glass sheet occupies when the glass is bent to substantially its desired shape. The mechanical means interrupts the free sliding of the peripheral portion that slides more rapidly than another portion of the periphery. Thus, mechanical interuption in sliding permits the slowest sliding marginal portion to catch up with the fastest sliding marginal portion and thus re-orients the glass in a horizontal plane when the reoriented glass is permitted to slide over the interruption and slide freely once again.